Specifically, according to Kirchhoff's law for an opaque material, a surface rendered highly absorbent in a predetermined wavelength range by its structuring is also a surface capable of efficiently emitting thermal radiation in the same wavelength range. Because of its optical properties, such a surface is made capable of emitting infrared IR radiation with a spectrum different from that of a black body, while promoting emission in wavelengths close to the structuring dimensions.
It is known that highly absorbent structures are also structures which make it possible to efficiently emit thermal radiation at the same wavelengths according to Kirchhoff's law for an opaque body.
These structures are therefore of great interest particularly in the near infrared range, for forming good thermal IR sources. This type of source may, for example, be used for IR gas sensors. In this case, a material constituting the IR source is heated to high temperature (several hundreds of degrees), and it emits its IR radiation which passes through a region comprising a material, such as a gas, the composition of which is desired to be known. This material generally absorbs the infrared IR radiation in a precise wavelength range, and an infrared IR detector therefore makes it possible to record the transmitted radiation. The desired concentration of material (generally a gas) can then be deduced. Sources emitting in the range of from 1 to 5 microns make it possible, for example, to detect carbon dioxide CO2, carbon monoxide CO, as well as volatile hydrocarbons.
In the particular example of the source of a gaseous carbon dioxide CO2 sensor, this gas absorbs particularly in the range 4-4.5 microns. This is the band generally desired for analysis of this gas.
The materials generally used for infrared IR emission are as follows: so-called “black” metals such as platinum, gold and aluminium which are rendered black by their porous deposition structure, surface structures obtained by a femtosecond laser, carbon nanotubes, textured silicon coated with metal.
The structures of surfaces obtained by a femtosecond laser are described, for example, in the article by A. Y. Vorobyev entitled “Colorizing metals with femtosecond laser pulses”, published in Applied Physics Letters 92, 041914 (2008).
Textured silicon coated with metal is described, for example, in the article by L. Müller et al. entitled “Infrared emitting nanostructures for highly efficient microhotplates”, published in Journal of Micromechanics and Microengineering, 24(2014) 035014.
More recent techniques are now seeking selective emission in terms of spectra for the fields of thermophotovoltaics, hyperspectral analysis, and filter-free NDIR sensors. According to these techniques, surfaces functionalized with quantum wells now allow laboratory production of sources whose radiation can be controlled in terms of wavelength and emission in a narrow wavelength band.
One example of these surface structures is described, for example, in the article by T. Inoue et al. entitled “Filter-free nondispersive infrared sensing using narrow-bandwidth mid-infrared thermal emitters”, published in Applied Physics Express, 7, 012103 (2014). These structures, however, are expensive to produce, with technical steps of the electron-beam lithography type. Furthermore, they are intended to be integrated into evacuated housings, which increases the complexity.
In terms of materials, the stacks often comprise alternations of dielectric and metallic materials (stacks of the Salisbury type, using quarter-wave layers) which make it possible to increase the absorption and therefore the emission in a precise wavelength range. These stacks have the drawback of being relatively thick, and of comprising metals which are generally sensitive to oxidation at high temperature. Photonic crystals have also been proposed. Lastly, plasmonics has made it possible to propose resonant metal structures, although their metals remain sensitive to oxidation at high temperature.
MIM (metal/insulator/metal) structures have also been developed, because they make it possible to tune an absorption/emission line relatively easily, and could make it possible to combine spectral and angular selectivity. An example of an MIM structure is described in the article by H. T. Miyazaki entitled “Ultraviolet-nanoimprinted packaged metasurface thermal emitters for infrared CO2 sensing”, published in Science and Technology of Advanced Materials, No 16, (2015) 035005.
These structures, on the other hand, still have the drawback of comprising metals which may be sensitive to oxidation when the structures are heated in air, or generally in a weak vacuum.
Despite interesting advances, particularly in the selective emission of structures constituting IR sources for gas sensors, there are still those which, as thermal sources, need to be heated to several hundreds of degrees Celsius in order to emit their infrared IR radiation. These structures often comprise metals, all of which are more or less sensitive to oxidation in the long or short term, or to inter-diffusion of the layers used. Evacuation of emitting sources, which is often recommended for reasons of reducing the losses of heat or of stability of the materials, is still not a solution which is easy to implement, and is relatively expensive. If use in air or a weak vacuum is implemented, the existing solutions will be limited by oxidation of the structures. Furthermore, even without an oxidizing medium, the quality of the optical interfaces created may be affected by the diffusion of elements under the effect of temperature, making the emission characteristics of the IR sources unstable over time.
It is this technical problem of temperature stability, particularly in an oxidizing medium, which the present invention resolves.